LOW TEMPERATURE, PRESSURELESS SINTERING OF SILICON NITRIDE

Abstract: A silicon nitride-based powder composition that yields sintered bodies having a density of at least 3.15g/cm<3> by pressureless sintering. The composition includes silicon nitride and an amount of bismuth oxide as a phase transition aid in addition to magnesium oxide, aluminium oxide, zirconium oxide and, optionally, silicon dioxide. The sintered bodies can be produced at temperatures of 1650 DEG C or less.

Description: LOW TEMPERATURE, PRESSURELESS SINTERING OF SILICON NITRIDE Background of the Invention The invention relates generally to pressureless sintering of silicon nitride (Si 3 N 4 ) compositions. It relates particularly to pressureless sintering of such compositions at temperatures of 1650° Centigrade (°C) or below. It relates more particularly to pressureless sintering at such temperatures of Si 3 N 4 compositions that include bismuth oxide (Bi 2 0 3 ) and, optionally, zirconium oxide (Zr0 2 ) as part of a sintering aid combination. Silicon nitride ceramics are recognized for their excellent mechanical and physical properties, including good wear resistance, low coefficient of thermal expansion, good thermal shock resistance, high creep resistance and high electrical resistivity. In addition, Si 3 N 4 ceramics resist chemical attack, particularly oxidation. These properties or attributes make Si 3 N ceramics particularly useful in a variety of wear and high temperature applications, such as cutting tools and parts in pumps and engines. Silicon nitride has two characteristics, covalent bonding and low diff usivity, that pose problems for those seeking to convert Si 3 N 4 powder to a high density part with little or no porosity. The problems are particularly severe in the absence of densif ication aids, applied pressure at elevated temperatures as in hot pressing or both. Hot pressing Si 3 N 4 powder or preformed parts (also known as greenware) typically yields simple rectangular or disc-shaped articles. These articles must then be converted to a desired shape by various procedures that may be costly, slow or quite difficult because dense Si 3 N 4 bodies have a high degree of hardness. These procedures, which include grinding, sawing and drilling, generally do not produce intricate shapes. Largely because of limitations such as cost and limited availability of shapes, many efforts focus upon pressureless sintering as a preferred densif ication route. US-A-4,264,547 discloses Si 3 N 4 -based sintering compositions containing Si 3 N 4 and no more than 6% by weight (wt%) of very finely ground magnesium oxide (MgO) and aluminum oxide (Al 2 0 3 ) as densif ication aids. The MgO and Al 2 0 3 are present in a weight ratio that lies between 10: 1 and 1 :3. Pressureless sintering conditions include heating in a nitrogen atmosphere for 2 to 20 minutes at 1650° to 1830°C. J. Barta et al., in "Pressureless Sintering of Silicon Nitride", Science of Ceramics. Vol. 1 1, pages 219-224 (1981), describe sintering combinations of Si 3 N , MgO, Al 2 0 3 and, optionally, silicon dioxide (Si0 2 ) under 1.3 atmospheres (0.13 MPa . at 1550°C and 1650°C for one hour. R. W. Dupon et al., in "Low-Temperature Route to Cordierite Ceramics Using a Reactive Liquid Phase Sintering Aid, Dense Body Preparation and Green Tape Fabrication", Materials Research Society Symposium Proceedings, Vol. 154, pages 351-356 (1989), teach that Bi 2 θ 3 is a useful flux for preparing a dense body of cordierite. They also teach that the microstructure of a sintered body formed from 92 wt% cordierite and 8 wt% Bi 2 0 3 has residual Bi 2 θ3 flux in discontinuous domains at grain boundaries and triple points. JP (Kokai) Number 2-263763 describes sintering a molded material consisting of either 0.5-20 wt% Bi 2 0 3 powder or a combination of 0.5-10 wt% Bi 2 0 3 powder and 0.5-10 wt% Al 2 0 3 powder and the balance Si 3 N powder at a temperature between 1600°C and 2100°C under an added pressure of 1 to 2,000 atmospheres (0.1 to 200 MPa). The Bi 2 θ 3 acts as a sintering aid and remains in the sintered material by forming a highly viscous glass phase located at grain boundaries. Summary of the Invention A first aspect of the invention is a Si 3 N -based sintering composition in powder form comprising: (a) Si3N 4 in an amount of from 90 to 98 percent by weight of composition; (b) a densification aid combination of MgO, AI 2 O3 and, optionally, Si0 2 , the combination being present in an amount of from 2 to 10 percent by weight of composition provided that combined amounts of MgO and AI 2 θ3 are greater than 2 but less than 6 percent by weight of composition, the combination having a composition that, when portrayed in a triaxial composition diagram showing interrelationships among MgO, Al 0 3 and total Si0 2 content, is defined and encompassed by a shaded area shaped as a pentahedron and bound by a line that connects point A to point B, point B to point C, point C to point D, point D. to point E and point E to point A; and (c) Bi 2 θ3 in an amount sufficient to accelerate phase transition aid from alpha (α)- to beta (β)-Si3N . As used herein, the phrase "amount sufficient to accelerate" means that transformation of α-phase Si 3 N in a sintering composition to β-phase Si 3 N 4 during sintering occurs more rapidly when such an amount of Bi 2 0 3 is present than when it is absent. In addition, it is believed that such an amount facilitates attainment of a density of at least 3.15 g/cm 3 when the Si3 powder in the sintering composition has an average particle size in excess of 2 micrometers (μm). If an amount of Zr0 2 is present, the amount is desirably less than 2 percent by weight of composition. A second aspect of the invention is a process of manufacturing sintered Si 3 N 4 bodies having a density of at least 3.15 g/cm 3 and a residual α-SΪ3N 4 phase content of less than (<) 10 wt%, based on total body weight. The process comprises heating a preformed body prepared from the sintering composition of the first aspect to a temperature of from 1550 C C to 1650°C in an atmosphere composed essentially of nitrogen at atmospheric pressure for a period of time sufficient to attain the density. A third aspect of the invention is a sintered S13N4 body prepared from the composition of the first aspect by the process of the second aspect. The sintered body has a residual Bi 2 θ3 content of from greater than (>) 0 to < one part per million parts (ppm) of densif ied or sintered body. Sintered bodies resulting from the sinterable compositions of the invention have excellent physical properties, such as wear resistance, that make them suitable for a variety of end use applications. A partial list of such applications includes tappet inserts, valve guides, cam follower rollers, clevis pins, wrist pins, cam lobes, automotive valves, pump seals, needle bearing elements, ball bearings, ball bearing races, bushings, can tooling, punch tooling, needle valve pins, valves and other parts used in chemical process environments, blast nozzles, agitator parts for use in corrosive environments, parts used for abrasive handling apparatus, parts used in cutting applications such as milling, turning and drilling, and parts used in metal drawing or forming applications such as wire drawing dies. Other suitable applications include those listed at column 2, lines 26-48 of US-A-4,945,073. Brief Description of the Drawing Fig. 1 is a triaxial composition diagram showing combinations of MgO, AI 2 θ3 and total Si0 2 that provide satisfactory sintered Si 3 N 4 bodies when combined with an amount of Si 3 N that is from 90 to 98 wt% , based on weight of composition, and Bi 2 θ3 as a phase transition aid. Description of Preferred Embodiments For purposes of this invention, any Si 3 N 4 powder, including crystalline forms of α-Si3N 4 and β-Si 3 N 4 , or noncrystalline amorphous S13N 4 , or mixtures thereof, will provide satisfactory results. The powder has a particle size that is desirably < 5 μm, preferably < 2 μm. The powder may have such a particle size either as synthesized or as a result of conventional size reduction operations. The latter include ball milling, attrition milling and jet milling. As a general rule, increases in Si 3 N 4 powder purity lead to corresponding improvements in physical properties of sintered bodies prepared from such powders. Some sources of Si 3 N 4 powder may, however, contain metallic and nonmetallic impurities. Some impurities may be tolerated, but the amounts are desirably as low as possible. For example, oxygen is present to some extent as Si0 2 , typically found as a surface coating on S.3N4 particles. An oxygen content of no more than 5 wt% yields satisfactory results. Other impurities, such as elemental silicon, carbon and metals are suitably within limits disclosed at column 4, lines 2-30 of US-A-4,883,776. Raw Si3N powder cannot be densif ied to densities approaching theoretical density in the absence of densif ication aids such as refractory oxides, or nitrides or a combination of oxides and nitrides. For purposes of the invention, compositions suitably comprise S13N 4 , MgO, AI2O3, Si0 2 , Bi 2 0 3 and, optionally, Zr0 2 . Components other than Si3N 4 may have a size that is >_ that of Si3 4 without adversely affecting properties of sintered bodies prepared from sinterable compositions of the invention. Bi 2 0 3 is used as a phase transition aid. It can also be called a catalyst because, when added to other components of a sintering composition, it performs its desired functions and then, in large part, volatilizes under pressureless sintering conditions. In other words, the latter conditions lead to removal or volatilization of a major portion of the Bi 2 θ3 in the compositions. The Bi 2 0 3 is believed to act as a flux early in a pressureless sintering cycle by enhancing diffusion of sintering or densification aids. This leads, in turn, to an accelerated dissolution of α-Si3N 4 and, subsequently, to an enhanced reprecipitation of β'-SiAION. Typical residual Bi 2 0 3 levels in a sintered body are > 0 ppm, but no more than 1 ppm, desirably no more than 300 parts by weight per billion parts by weight of sintered body (ppb) and preferably no more than 200 ppb, following sintering. Residual Bi 2 0 3 levels may be determined by grinding a sintered body into particles having a size of < 45 μm, dissolving the particles in an acid mixture (50/50, by volume, concentrated HF/concentrated HNO3) and then analyzing the acid solution for bismuth content via inductively coupled plasma (ICP) mass spectrometry. ICP mass spectrometry is discussed by Montasser et al. in Inductively Coupled Plasmas in Analytical Atomic Spectrometry, Second Edition, VCH Publishers,(1992) and by Tan et al. in Applied Spectroscopy, 40(4), pages 445-60, (1986). The compositions desirably have a Bi 2 θ3 content within a range of from 0.1 to 2 wt%, based upon composition weight. The range is preferably from 0.1 to 1.25 wt% . If the content is < 0.1 wt%, sintered body residual α-Si3N 4 phase content tends to be higher and toughness tends to be lower than comparable properties of sintered bodies having a Bi 2 0 3 content within the range. Amounts in excess of 2 wt% are not needed. Although sintering does occur with amounts in excess of 2 wt%, weight losses due to volatility of the Bi 2 θ3 may be undesirable. The addition of Bi 2 0 3 is believed to lead to improvements in toughness without adversely affecting other properties such as hardness relative to sintering compositions that are identical save or the absence of Bi 2 θ 3 . The addition of Bi 2 θ3 is also believed to provide these improvements without substantial grain growth. The minimal grain growth appears to arise from an ability to achieve α-SJ3N 4 to β'-SiAION transformation under less severe conditions, such as temperature and time at temperature, than are needed to attain a density of 3.15 g/cm 3 in the absence of Bi 2 θ3. An advantage of limited grain growth is that it facilitates attainment of very fine surface finishes. These finishes are believed to lead, in turn, to enhanced wear properties. The MgO and AI 2 θ3 are desirably present in a combined total amount of > 2, but < 6 wt%, based on composition weight. Figure 1 shows interrelationships among MgO, AI 2 θ 3 and total Si0 2 content. Densif ied Si 3 N 4 bodies having a density ^ 3.15 g/cc are readily obtained from combinations of MgO, AI 2 θ3 and total Si0 2 content that lie on or within lines connecting point A to point B, point B to point C, point C to point D, point D to point E and point E to point A thereby forming pentahedron ABCDE. Component amounts, in terms of wt%, based upon combined amounts of MgO, AI 2 O3 and total Si0 2 , for each point are as follows: pointA- 25 wt% MgO, 24 wt% Al 2 0 3 and 51 wt% total Si0 2 ; point B - 17 wt% MgO, 34 wt% Al 2 0 3 and 49 wt% total Si0 2 ; point C - 24 wt% MgO, 58 wt% Al 2 0 3 and 18 wt% total Si0 2 ; point D - 41 wt% MgO, 41 wt% Al 0 3 and 18 wt% total Si0 2 ; and point E - 49 wt% MgO, 21 wt% Al 2 0 3 and 30 wt% total Si0 2 . Total Si0 2 content is made up of oxygen in the Si 3 N powder (assumed to be present as Si0 2 ) and any Si0 2 additions. A Si3N powder having an α- Si 3 N content of > 90 wt% is believed to have a minimum oxygen content of 0.8 wt%, based on powder weight. This converts to an approximate minimum total Si0 2 content of 18 wt% for points D and E. In a typical process, Si3N 4 powder and other components of the compositions are milled together using conventional apparatus such as ball mills, attrition mills, jet mills and vibratory mills. The compositions may be milled or blended either by wet or dry milling. Wet milling using an attrition mill and a carrier medium yields satisfactory results. Suitable carrier media and amounts thereof are listed at column 6, line 62 through column 7, line 17 of US-A- 4,883,776. The carrier medium is preferably water, an alcohol such as ethanol, or a ketone. After milling, the blended powder is dried, recovered, pressed into a desired shape and densif ied. Drying employs standard means such as spray drying or oven drying under a nitrogen purge. Subsequent to drying, the powder is beneficially sieved through a screen to obtain a powder having a desirable size. Cold isostatic pressing, particularly when preceded by uniaxial cold pressing, provides satisfactory shaped articles. Suitable pressures for such pressing are 5,000 pounds per square inch (psi) (35 MPa) for uniaxial cold pressing and 35,000 psi (245 MPa) for cold isostatic pressing. Shaped articles made from the compositions described above are preferably densified by pressureless sintering. Other conventional densif ication techniques such as hot pressing and hot isostatic pressing may also be used if desired. Densif ication desirably occurs in the presence of a nitrogen atmosphere at a temperature of from 1550° to 1650°C. At temperatures below 1550°C, it is difficult, if not impossible, to attain a density of at least 3.15 g/cm 3 in a commercially reasonable time. At temperatures in excess of 1650°C, sintering occurs, but advantages of low temperature sintering are lost. At temperatures in excess of 1800 C C, Si3N begins to decompose under pressureless sintering conditions. The resultant densified articles desirably have a density of at least 3.15 g/cm 3 . Sintering can occur with crucibles and furniture that are fabricated from either AI 2 θ3 or boron nitride (BN). The crucible and furniture may also be fabricated from a ceramic material other than BN and then coated with BN. It can also occur in an oxide-based furnace in the presence of flowing nitrogen gas. If a binder is used to convert the powdered sintering composition into ceramic greenware or a cast tape, an additional step may be added prior to sintering. In this step, either the uncompacted powder or the greenware or tape is heated at moderate temperatures such as from 500° to 1 ,000°C in order to remove volatile binder materials. The same temperatures may be sufficient to partially react the starting ceramic materials. Mechanical properties of sintered bodies are readily measured by use of standard tests. Fracture toughness (K|c) is measured by the Chevron notch technique. Fracture strength (modulus of rupture) is measured according to Military Standard 1942b or Japanese Industrial Standard (JIS) 1601. Military Standard 1942b, which incorporates a 4-point bend test, is explained in detail at column 10, line 57-68 of US-A-4,883,776. Hardness is measured according to the Vickers indentation test which is explained at column 11 , lines 43-64 of US-A-4,883,776. Density is determined by the Archimedes method. Alpha- and β-SiAION concentrations in sintered bodies are determined by powder x-ray diffraction (XRD) of crushed samples. The following examples illustrate, but do not limit, the invention. Unless otherwise stated, all parts or percentages are based upon weight. Example 1 A powdered composition was prepared from: 100 grams (g) Si3N 4 powder, (Ube Industries, SN-E10), having an average particle size (diameter) of 0.6 μm and a nominal BET surface area of 11 m2/g; 0.78 g fumed Si0 2 (Cabot Corp., CABOSIL'" L90); 2.82 g Al 2 0 3 (Aluminum Company of America, A- 16 Superground); 2.78 g MgO (Fisher Scientific); and 0.58 g Bi 2 θ3 (Johnson-Matthey, 99.99 % purity). The composition was attrited for one hour at 350 revolutions per minute (rpm) using a Union Process (Model 1S) heavy duty lab attritor, 3/16 inch (0.48 cm) yttria stabilized zirconia (YSZ) media and 225 cm 3 of denatured ethanol as a carrier medium. The attrited composition was dried under flowing nitrogen at a temperature of 60°C 'for a period of 18 hours to provide a mixture of dried powder and milling media. The media was separated from the dried powder by screening through a 60 mesh (Tyler equivalent) (250 μm sieve aperture) stainless steel screen. A 10 gram (g) quantity of the dried powder was converted to a pressed part by pressing the powder in a 1 inch (2.5 cm) diameter steel die at a pressure of 5,000 psi (34.5 MPa). The pressed part was then placed in a rubber bag that was evacuated and then wet bag isopressed at 36,000 psi (248 MPa) to provide sinterable samples. Wet bag isopressing is described by J. S. Reed in Introduction to the Principles of Ceramic Processing at page 349, John Wiley & Sons, New York (1987). The samples were sintered at 1610°C for 4 hours in a BN crucible under flowing nitrogen at atmospheric pressure using a heating rate of 20°C per minute. The sintered samples had a fracture strength (Military Standard 1942b) of 721 MPa, a hardness (Vickers with a 10.3 kg load) of 1550 kg/mm 2 , a density of 3.16 g/cm 3 and a fracture toughness (Chevron notch) of 6.64 MPaem 1 ^. Examples 2 - 15 The procedures of Example 1 were repeated through separation or recovery of the dried powder. Eighty gram quantities of the dried powder were pressed into a 1 inch (2.5 cm) by 6 inch (15 cm) steel die at a pressure of 4,000 psi (27.6 MPa) to provide pressed parts that were then wet bag isopressed as in Example 1 , placed in enclosed BN crucibles (Carborundum AX05) and sintered in the presence of flowing nitrogen. Sintering occurred in a furnace using the following schedule: heat at 20°C/minute from ambient temperature to 1425°C; hold at 1425°C for 30 minutes; heat at 20°C/minute up to 1575°C; hold at 1575°C for 300 minutes; heat at 20°C/mιnute up to 1625°C; and hold at 1625°C for 60 minutes. The crucible and its contents were removed from the furnace after cooling to ambient or room temperature (taken as 25°C). Powder compositions, wt% of MgO, wt% of AI2O3 and total wt % of Si0 2 (based upon combined weight of MgO, AI2O3 and total Si0 2 ), amount of Siθ 2 present on Si3 4 powder, Vickers Hardness and density measurements of the resultant sintered parts are shown in Table I. The sintered parts had a residual Bi2θ3 content, measured by ICP as described herein, of 50 to 300 ppb. LO t o υ. o TABLE 00 Not an example o the Invention = Based on combined weight of MgO, Al 2 0 3 and Total Si0 2 - = Not Measured M LΠ O TABLE I I LO I = Not an exampe o t e Inventon = Based on combined weight of MgO, Al 2 0 3 and Total Siθ2 - = Not Measured The data presented in Examples 2- 15 demonstrate several points. First, sintering compositions do not yield a sintered density of at least 3.15 g/cm 3 unless two conditions are met. One is that the MgO, AI2O3 and total Siθ2 weight percentages, based on combined weight thereof, must fall inside the pentahedron shown in the Figure. The second condition is that MgO and Al2θ 3 must be present in a combined amount of > 2 wt%, but < 6 wt%, based on total sintering composition weight. Second, Siθ2 addition is optional as evidenced by Examples 4 and 5 so long as the aforementioned conditions are met. Third, the addition of a large amount of additional Si0 2 does not provide a desired density even though total densif ication aid concentration is increased. Examples 16-28 Examples 16-28 were prepared by repeating the procedure used for Examples 2- 15. Table II lists compositions used in preparing sintered parts as well as toughness (Chevron notch), strength, Vickers Hardness and density of the resultant sintered parts. LO LO K> Π O L/l O TABLE II (Parti) = Not an exampe o t e Inventon = Based on combined weight of MgO, AI 2 O3 and Total Si0 2 -- = Not Measured l M LΠ u o. VI o TABLE II (Part i) r I = Based on combined weight of MgO, Al 2 0 3 and Total Si0 2 -- = Not Measured l tsl o LΠ O = Not an exampe o t e Inventon = Not Measured _J K. o o The data in Table II demonstrate the advantageous effect that Bi 2 θ3 has on the sintering of Si 3 N in the MgO-Al2θ 3 -total Si0 2 densification aid system. Example 25 shows that full conversion to β'-SiAION is possible with low Bi 2 θ3 additions (0.12 wt%). Example 26 shows that even very low Bi2θ3 concentrations cause almost complete α-SJ3N 4 to β'-SiAION 5 transformation (4 wt% residual α-Si 3 N ) under conditions specified or the example. In the absence of B.2O3, as shown in Examples 27 and 28, residual α-Si 3 N concentrations exceed 10 wt% under the same conditions. Incomplete conversion leads, in turn, to lower toughness, a generally undesirable trend. The presence of Bi2θ3 thus improves toughness without excess reduction of strength or a large increase in grain size. Compare, for example, Examples 17 and 10 27 wherein toughness increases by 65% whereas strength decreases by only 17% and hardness decreases by 8% . At the same time grain size only increases from 0.17 μm ( + /- 0.08 μm) to 0.28 μm (+ /- 0.15 μm). In other words, the addition of Bi2θ3 in sufficient quantities accelerates the α-Si3N 4 to β'-SiAION transformation under a given set of sintering conditions and enhances toughness without an undesirable level of decrease in other desirable physical properties. In 15 addition, only trace amounts of Bi 2 0 3 remain in sintered bodies. This means that desirable improvements are attainable with very little composition modification. Examples 29-34 Effect of ZrO? Addition Examples 29-34 were prepared by repeating, with certain exceptions, the procedure of Examples 2-15. Examples 29, 31 and 32 used Si 3 N 4 milling media rather than Zr0 2 20 milling media. This effectively excluded Zr0 2 from the sintered bodies. The use of Zr0 2 milling media typically resulted in the addition of 0.25 to 0.75 wt% of Zr0 to the compositions used in making the sintered bodies. Examples 29 and 30 used the sintering program of Examples 2-15 save for extending the hold time at 1625°C from 60 minutes to 180 minutes and eliminating the steps of holding at 1425°C and 1575°C. Examples 31 and 33 used the same sintering program as 25 Examples 29 and 30 save for reducing the sintering temperature from 1625°C to 1600°C. Examples 32 and 34 used the sintering program of Examples 29 and 30 save for increasing the temperature from 1625°C to 1650°C. In addition, Examples 29 and 30 used a Si3N 4 powder having a BET surface area of 19.5 m 2 /g (Starck, LC-12N) rather than the Si3N powder of Example 1. Table III shows compositions used to make sintered parts and density of the 30 resultant sintered parts. 35 TABLE III (Part i) * = Not an example of the Invention N/A = Not Applicable -- = Not Measured TABLE III (Part 2) * = Not an example of the Invention N/A = Not Applicable -- = Not Measured = Based on Combined weight of MgO, Al 2 0 3 and Total Si0 2 The data in Table III illustrate several points. First, a comparison of Examples 29 and 30 demonstrates the usefulness of Zr0 2 in attaining a density of at least 3.15 g/cm 3 . Second, Examples 31 -34 again show that Siθ2 additions are not necessary to attain such a density. Third, Examples 31-34 show that Si3N 4 powders having no particles > 2 μm do not require ZrU2 to attain such a density. With larger particle sizes, Zr0 2 additions help promote densif ication. Thus, densif ication aid combinations within the MgO-AI 2 θ3-total Siθ 2 pentahedron of the Figure require only small amounts of Bi2θ3 for development of sintered bodies with high toughness and minimal residual α-Si 3 N 4 concentration, provided that MgO and AI2O3 are present in a combined amount of > 2 wt%, but < 6 wt%, based on total sintering composition weight. Although this series of examples incorporated Zr0 2 by way of the milling media, similar results are expected when powdered Zrθ 2 is added with the other powdered components in preparing the sintered bodies. Examples 35-47 - Effect of Starting SUN_ι Powder Examples 35-47 were prepared by repeating the process used for Examples 2-15 save for varying the highest temperature used in sintering, the time at that temperature and eliminating the steps of holding at temperatures of 1425°C and 1575°C. In addition, Examples 35-46 used S13N4 powders other than that used in Examples 1-15. Example 47 used the same S13N4 powder, designated as "A", as in Examples 1-15. It had a nominal average particle diameter (d50) of 0.6 μm, a maximum particle size of 2 μm and contained 100% α-SJ3N 4 . Powder "B" was commercially available from Shinetsu under the trade designation KSN 10SP. It had a d50 of 0.6 μm, a maximum particle size of 2 μm and a BET surface area of 13 m 2 /g, and it contained 94% α-S.3 . Powder "C" was commercially available from Shinetsu under the trade designation KSN 20SP. It had a d50 of 0.9 μm, a maximum particle size of 4.8 μm and a BET surface area of 12.8 m 2 /g, and it contained 79% α-SJ3N . Powder "D" was commercially available from Shinetsu under the trade designation KSN 80SP. It had a d50 of 0.6 μm, a maximum particle size of 4 μm and a BET surface area of 10 m /g, and it contained 17% α-Si3 . Powder "E" was commercially available from Shinetsu under the trade designation KSN 10M-TX. I had a d50 of 1.2 μm, a maximum particle size of 13.7 μm and a BET surface area of 8 m 2 /g, and it contained 94% α-Si 3 N . Table IV lists the components used to make Examples 35-47, identifies the Si 3 N 4 source, provides the sintering temperature and time at temperature, and shows the density of resulting sintered bodies. TABLE IV (Part 2) * = Not an example of the Invention ** * = Based on Combined weight of MgO, AI 2 θ3 and Total SiQ 2 The data presented in Table IV illustrate several points. First, a comparison of Examples 35, 37, 39, 41 , 43 and 45 shows that higher sintering temperatures or longer times at a sintering temperature may be needed for certain powders in order to obtain a density of at least 3.15 g/cm 3 . Second, Examples 36, 40 and 44 suggest that S13N 4 powders having a comparatively large maximum particle size (13.7 μm) are not conducive to realizing a density of 3.15 g/cm 3 or greater. The remaining Examples in Table IV show that Si 3 N 4 powders having smaller maximum particle sizes (on the order of 4.8 μm or less) can yield a density of at least 3.15 g/cm 3 . Other Si3 powders that have a maximum particle size in excess of 4.8 μm, but < 13.7 μm, should also provide such a density provided sufficient Bi 2 θ3, AI2O3 and MgO are present and sintering temperatures and time at sintering temperature are optimized. Fourth, Examples 38, 42 and 46 show that Si 3 N powders having a relatively low α-Si 3 N 4 content and a correspondingly high β-SJ3N 4 content can yield a satisfactory density. Fifth, Example 47 shows that it is possible to prepare a sintered body with a density of at least 3.15 g/cm 3 at temperatures below 1600°C. Similar results are expected with other Si3N powders, combinations of sintering time, sintering temperature and component amounts, all of which are disclosed herein.

Claims: CLAIMS: 1. A silicon nitride-based sintering composition in powder form comprising (a) silicon nitride in an amount of from 90 to 98 percent by weight of composition; (b) a densification aid combination of magnesium oxide, aluminum oxide and, optionally, silicon dioxide, the combination being present in an amount of from 2 to 10 percent by weight of composition provided that combined amounts of magnesium oxide and aluminum oxide are greater than 2, but less than 6, percent by weight of composition, the combination having a composition that, when portrayed in a triaxial composition diagram showing interrelationships among magnesium oxide, aluminum oxide and total silicon dioxide content, is defined and encompassed by a shaded area shaped as a pentahedron and bound by a line that connects point A to point B, point B to point C, point C to point D, point D to point E and point E to point A; and (c) bismuth oxide in an amount sufficient to accelerate phase transition aid from alpha- to beta-silicon nitride. 2. A composition as claimed in Claim 1, wherein the silicon nitride has a particle size that is less than or equal to 5 μm and each component of the composition other than silicon nitride has a particle size that is greater than that of silicon nitride. 3. A composition as claimed in Claim 2, wherein the silicon nitride has a particle size that is less than or equal to 2 μm. 4. A composition as claimed in Claim 1 or Claim 3, wherein the densification aid combination also includes zirconium oxide in an amount that is greater than or equal to 0 but less than 2 percent by weight of the composition. 5. A composition as claimed in Claim 1 or Claim 4, wherein the amount of bismuth oxide is from 0.1 to 2 percent by weight of the composition. 6. A process for manufacturing a sintered silicon nitride body having a density of at least 3.15 g/cm 3 and an alpha-silicon nitride content of less than 10 percent by weight based upon weight of silicon nitride in the body, the process comprising heating a preformed body to a temperature within a range of from 1550°C to 1650°C in an atmosphere composed essentially of nitrogen at atmospheric pressure for a period of time suf icient to attain the density, the body being prepared from a composition comprising (a) silicon nitride in an amount of from 90 to 98 percent by weight of composition; (b) a densification aid combination of magnesium oxide, aluminum oxide and, optionally, silicon dioxide, the combination being present in an amount of from about 2 to about 10 percent by weight of composition provided that combined amounts of magnesium oxide and aluminum oxide are greater than 2, but less than 6, percent by weight of composition, the combination having a composition that, when portrayed in a triaxial composition diagram showing interrelationships among magnesium . oxide, aluminum oxide and total silicon dioxide content, is defined and encompassed by a shaded area shaped as a pentahedron and bound by a line that connects point A to point B, point B to point C, point C to point D, point D to point E and point E to point A; and (c) bismuth oxide in an amount sufficient to accelerate phase transition from alpha- to beta-silicon nitride. 7. A process as claimed in Claim 6, wherein the densification aid combination also includes zirconium oxide in an amount that is greater than or equal to 0, but less than 2, 5 percent by weight of the composition. 8. A process as claimed in Claim 6, wherein the period of time is from 10 minutes to 12 hours. 9. A process as claimed in any of Claims 6-8, wherein heating the preformed body causes removal of a major portion of the bismuth oxide. 0 10. A process as claimed in Claim 9, wherein sufficient bismuth oxide is removed to provide the sintered body with a residual bismuth oxide content of greater than 0, but less than 1 , part by weight per million parts by weight of sintered body. 11. A process as claimed in Claim 10, wherein the residual bismuth oxide content is less than 300 parts by weight per billion parts by weight of sintered body. 15 12. A sintered silicon nitride body with a density of at least 3.15 g/cm 3 , and an alpha-silicon nitride content of less than 10 percent by weight based upon weight of silicon nitride in the body and a residual bismuth oxide content of from greater than 0 to less than one part per million parts (ppm) of densified body. 13. A sintered body as claimed in Claim 12, wherein the residual bismuth oxide '20 content is less than 300 parts by weight per billion parts by weight of sintered body. 25 30 35

Inventors: NILSEN KEVIN J, NILSEN, KEVIN, J.
Applicants: DOW CHEMICAL CO, THE DOW CHEMICAL COMPANY
Application year: 1994
Main classification: C04B35
IPC codes: C04B35